Communication relay apparatus, information management system, and control method and program therefor

ABSTRACT

A communication relay apparatus, information management system, and control method and program therefor. The communication relay apparatus includes several communication ports, and that includes a communication relay section, a buffer, a control signal transmitting section, a bandwidth information acquiring section, and a transmission interval control section.

CROSS-REFERENCE TO RELATED APPLICATION

The present application claims priority to Japanese Patent ApplicationNumber 2005-077400, filed with the Japan Patent Office on Mar. 17, 2005.

FIELD OF THE INVENTION

The present invention relates to a communication relay apparatus, aninformation management system, and a control method and programtherefor. Particularly, the present invention relates to a communicationrelay apparatus that provides congestion control, an informationmanagement system, and a control method and a program therefor.

BACKGROUND OF THE INVENTION

A system has been put into practical use wherein servers or disk drivesare connected by fiber channels, via a broad-band network, to createdata backups in remote areas. According to this system, since databackups can be stored in remote locations that are less affected bynatural disasters occurring at local areas, the possibility that datawill be lost is extremely low. In this system, the bandwidth of thebroad-band network is smaller than the bandwidth of a local network, andcongestion may occur at a switching device that serves as a relay forthe broadband network and the local network.

Conventionally, to prevent such congestion, a switching device is usedthat employs buffer credit for the performance of flow control (see, forexample, Japanese Patent Publication number 2000224180A). According tobuffer credit control, a notification is transmitted to a disk driveconnected to the switching device concerning the remaining capacity of acommunication data buffer provided for the switching device, and uponreceiving this notification, the disk drive adjusts the transmission offrames so as not to exceed the number of credits. Thus, buffer overflowat the switching device can be prevented.

SUMMARY OF THE INVENTION

When a plurality of disk drives are connected to a switching device, auser may desire to allocate communication priorities in accordance withimportance levels established for the individual disk drives. Forexample, when a disk drive on which data for the main businessactivities are recorded and another disk drive on which information tobe provided is recorded are connected to the switching device,communication with the disk drive on which data for the main businessactivities are recorded should have the higher priority. On the otherhand, when a communication channel has a satisfactorily large bandwidth,it should be used as fully as possible for communication, regardless ofwhich disk drive has the higher priority.

Furthermore, as it is known in the art, information related tocongestion must be transmitted by a device that detected the congestionto other devices one after another. Thus, between these devices, a newdata frame must be defined for the transmission of congestioninformation, and the common use with the conventional technique is low.

Therefore, certain objects of the present invention are to provide acommunication relay apparatus that can resolve the above describedproblems, an information management system and a control method and aprogram therefor. These objects can be achieved by combinations of thefeatures described in the independent claims of the present invention.The dependent claims of the invention define specific examples that areadditional advantages of the present invention.

To achieve these objects, according to one aspect of the presentinvention, there is provided a communication relay apparatus having aplurality of communication ports, that comprises:

a communication relay section, including at least one receiving sidecommunication port which receives communication data from a transmissionside apparatus and at least one transmitting side communication portwhich transmits the communication data to a receiving side apparatus,the communication relay section transmitting the communication datareceived from the transmission side apparatus through the receiving sidecommunication port to a receiving side apparatus through thetransmitting side communication port;

a buffer being connected with the transmission side communication portfor storing the communication data before the transmission to thereceiving side apparatus through the transmitting side communicationport;

a control signal transmitting section for transmitting, in response to atransmission of the communication data from the buffer, a control signalrepresenting a status in which an entry of the buffer is available tothe transmitting side apparatus at a predetermined transmissioninterval;

a bandwidth information acquiring section for acquiring the bandwidth ofthe communication line connecting to the transmission side communicationport; and

a transmission interval control section for controlling the transmissioninterval in accordance with the bandwidth acquired by the bandwidthinformation acquiring section. Also, a method for controlling thecommunication relay apparatus, a program for controlling thecommunication relay apparatus, and an information management systemincluding the communication relay apparatus are provided.

The overview of the invention does not enumerate all the featuresrequired for the invention, and sub-combinations of these features canalso constitute the present invention.

According to the present invention, when a communication relay apparatusis connected to multiple communication lines having differentcommunication bandwidths, congestion occurs less frequently.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing the general configuration of a communicationnetwork.

FIG. 2 is a functional block diagram showing a storage device.

FIG. 3 is a functional block diagram showing a communication relayapparatus.

FIG. 4 is a functional block diagram showing a controller.

FIG. 5 is a time chart showing communication performed between storagedevices and the communication relay apparatus.

FIG. 6 is a flowchart showing the processing performed by the controllerfor setting a transmission interval.

FIG. 7 is a graph showing a transmission interval the controller changesin accordance with the number of vacant entries in an output buffer.

DETAILED DESCRIPTION OF THE INVENTION

The present invention will now be described by referring to oneembodiment. However, the following embodiment imposes no limitations onthe invention described in the claims, and not all combinations offeatures explained in the embodiment are requisite for the invention.

The general configuration of a communication network 10 is shown inFIG. 1. The communication network 10 includes: an information managementsystem 15-1 and an information management system 15-2, which aremutually connected via a broadband network 15-3. The informationmanagement system 15-1 includes a storage device 20-1, a storage device20-2, a communication relay apparatus 30-1 and a management computer 35.The communication relay apparatus 30-1 connects the storage device 20-1and the storage device 20-2 to the broadband network 15-3 usingcommunication lines that conform, for example, to fiber channelstandards. The management computer 35 reads, from a recording mediumsuch as a CD-ROM 40, a program that provides control for thecommunication relay apparatus 30-1, which is permitted to record orexecute the program. The information management system 15-2 includes astorage device 20-3, a storage device 20-4 and a communication relayapparatus 30-2. The communication relay apparatus 30-2 connects thestorage device 20-3 and the storage device 20-4 to the broadbandnetwork.

The storage devices 20-1 and 20-2 transmit stored data to the storagedevices 20-3 and 20-4 for recording therein, and when a failure occursat either the storage device 20-1 or 20-2, to recover data, the storagedevices 20-3 and 20-4 transmit the stored data to either the storagedevice 20-1 or 20-2. In this manner, according to the communicationnetwork 10 of this embodiment, since the storage devices 20-3 and 20-4are employed as backups, in the event a failure occurs, loss of data canbe prevented.

While taking into consideration large disasters, such as naturaldisasters, it is preferable that the information management system 15-1be located at a remote area, far distant from the information managementsystem 15-2. In this case, since the cost of long-distance communicationvia the broadband network is increased, the bandwidth employed tends tobe narrow. On the other hand, since the information management system15-1 is installed, for example, in an office building, the communicationline between the storage device 20-1 and the communication relayapparatus 30-1 is shorter and has a larger bandwidth than thecommunication line for the broadband network. Thus, the communicationrelay apparatus 30-1 is connected to two types of communication lines,for which the communication bandwidths differ greatly.

In such a case, when communication data from the storage device 20-1 or20-2 are transferred at an extremely high speed, the communication relayapparatus 30-1 can not immediately transmit these received data to thebroadband network. As a result, data overflows at an output buffer 340provided for the communication relay apparatus 30-1, and congestionoccurs. The communication relay apparatus 30-1 of this embodiment isprovided in order to effectively prevent such congestion.

The functional blocks of the storage device 20-1 are shown in FIG. 2.The storage device 20-1 includes a recording section 200, which is usedto record data, and a transmission section 210. The transmission section210 receives a control signal indicating that an entry in the outputbuffer 340 in the communication relay apparatus 30-1 is vacant, and thentransmits, to the communication relay apparatus 30-1, data stored in therecording section 200. Thereafter, the data recorded in the recordingsection 200 are sequentially transmitted to the transmission section 210and are then transmitted to the storage device 20-3 or 20-4. As aresult, data stored in the recording section 200 can be backed up toeither of the storage devices 20-3 and 20-4.

The functional blocks of the communication relay apparatus 30-1 areshown in FIG. 3. The communication relay apparatus 30-1 includes: acommunication relay section 300, the output buffer 340, a buffer monitorsection 350, a setup information memory 360, and a controller 370. Thecommunication relay section 300 acts as a communication relay betweenthe storage devices 20-1 and 20-2 and the storage devices 20-3 and 20-4.Specifically, first, the communication relay section 300 receivescommunication data from the storage devices 20-1 and 20-2 throughreception ports 310-1 and 310-2. Then, the communication relay section300 transmits the received communication data, through a transmissionport 320, to one of the storage devices 20-3 and 20-4 on the receptionside.

More specifically, the reception port 310-1 is connected to the storagedevice 20-1, and includes a receiver 312-1 and a transmitter 314-1. Thereceiver 312-1 stores, in the output buffer 340, communication datareceived from the storage device 20-1. The transmitter 314-1 transmits acontrol signal to the storage device 20-1 in accordance with aninstruction received from the controller 370, and the reception port310-2 is connected to the storage device 20-2 and includes a receiver312-2 and a transmitter 314-2. The receiver 312-2 stores, in the outputbuffer 340, communication data received from the storage device 20-2,and the transmitter 314-2 transmits a control signal to the storagedevice 20-2 in accordance with an instruction received from thecontroller 370.

The transmission port 320 is connected to the broadband network andincludes a transmitter 325, and the transmitter 325 reads communicationdata from the output buffer 340 and transmits the communication data tothe broadband network. The output buffer 340 stores the communicationdata, which are received through the reception port 310-1 or 310-2 andare to be transmitted through the transmission port 320, and the buffermonitor section 350 detects the number of vacant entries in the outputbuffer 340 and transmits this count to the controller 370. Thereafter,the setup information memory 360 records a variety of informationrelated to congestion control, such as the communication bandwidth of abroadband network, a reference value for the number of vacant entriesused to detect congestion, and the maximum quantity of communicationdata that can be sequentially transferred to the communication relayapparatus 30.

The controller 370 controls intervals for the transmission of controlsignals from the reception ports 310-1 and 310-2.

The functional blocks of the controller 370 are shown in FIG. 4. Thecontroller 370 includes a notification section 400, a congestiondetermination section 410, a bandwidth acquisition section 420, atransmission interval control section 430 and a control signaltransmission section 440. The notification section 400 provides for thestorage devices 20-1 and 20-2 a predesignated value, smaller than thecapacity of the output buffer 340, that is the maximum quantity ofcommunication data that can be sequentially transmitted to thecommunication relay apparatus 30-1.

On condition that the number of vacant entries in the output buffer 340becomes smaller than a predesignated reference value, the congestiondetermination section 410 determines that this is the result ofcommunication congestion in the communication relay apparatus 30. Thebandwidth acquisition section 420 then obtains the bandwidth of acommunication line connected to the transmission port 320, i.e., thebandwidth of the broadband network. As the bandwidth, the bandwidthacquisition section 420 may obtain a value previously stored in thesetup information memory 360, or may receive a bandwidth notificationfrom an external device for performing bandwidth detection.

Based on the bandwidth obtained by the bandwidth acquisition section420, the transmission interval control section 430 controls theintervals at which the control signal transmission section 440sequentially transmits multiple control signals for a plurality ofvacant entries. For example, when the bandwidth is 1 Mbps and thecapacity of one entry is 100 kbit, the transmission interval controlsection 430 divides 100 k by 1 M, and designates a transmission intervaltiming of 0.1 second.

On condition that the communication relay section 300 has transmittedcommunication data from the output buffer 340, at a predeterminedtransmission interval designated by the transmission interval controlsection 430, the control signal transmission section 440 transmits, tothe storage device 20-1 or 20-2, a control signal indicating an entry inthe output buffer 340 is vacant.

FIG. 5 is a time chart for communication between the storage devices20-1 and 20-2 and the communication relay apparatus 30-1. On conditionthat the storage devices 20-1 and 20-2 are accessed, the notificationsection 400 of the communication relay apparatus 30-1 provides for thestorage devices 20-1 and 20-2 the maximum quantity of communication datathat can be sequentially transferred (BCC=7). Preferably, the maximumdata quantity is smaller than the maximum capacity of the output buffer340, and in this embodiment, in FIG. 5, is set to 7, which is smallerthan 16, the maximum capacity of the output buffer 340. With thisarrangement, the occurrence can be prevented of a phenomenon whereincongestion is detected within an extremely short period of time andcommunication control is disabled.

On condition that circumstances are those whereunder there is nocongestion, communication data are sequentially transmitted from thestorage device 20-1 to the communication relay apparatus 30-1 atpredesignated communication intervals (e.g., about several μ seconds).On the other hand, in circumstances whereunder there is congestion, oncondition that communication data has been transmitted from the outputbuffer 340, the communication relay apparatus 30-1 transmits to thestorage device 20-1 or 20-2 a control signal indicating an entry for theoutput buffer 340 is now vacant. This control signal is a R_RDY signal,for example, for a fiber channel. In response to receipt of this signal,the storage device 20-1 or 20-2 transmits communication data, as a dataframe, to the communication relay apparatus 30-1.

Communication priorities are thereafter allocated to the reception ports310-1 and 310-2 connected respectively to the storage devices 20-1 and20-2. For example, in this embodiment, a higher priority is allocatedthe reception port 310-2 connected to the storage devices 20-2 than isallocated the reception port 310-1 connected to the storage devices20-1. The priority rating allocated reflects, for example, a QOS(Quality of Service) level. More specifically, priority ratings may bedefined based on relative and qualitative references, such as high,middle and low levels, or based on a quantitative reference, such as anaverage data rate or a guaranteed data rate. In any event, the controlsignal transmission section 440 of the communication relay apparatus30-1 transmits a control signal to the reception port having the higherpriority at a transmission interval that is shorter than that for thereception port having the lower priority. For example, within the periodof time represented in FIG. 5, an R_RDY signal is transmitted to thestorage device 20-1 only twice, while an R_RDY signal is transmitted tothe storage device 20-2 four times.

FIG. 6 is a flowchart showing the processing performed by the controller370 when setting a transmission interval. Following START, the buffermonitor section 350 detects the number of vacant entries in the outputbuffer 340 (S500). Then, the congestion determination section 410determines whether the number of vacant entries in the output buffer 340is smaller than a reference value, and decides whether communicationperformed by the communication relay apparatus 30 has become congested(S510). On condition that congestion is not detected (NO at S510), thetransmission interval control section 430 sets a predesignatedtransmission interval (e.g., several μ seconds to several hundreds of μseconds) that is shorter than the transmission interval that isdesignated on condition that congestion is detected (S520).

On condition that congestion is detected (YES at S510), the bandwidthacquisition section 420 obtains the bandwidth of a communication lineconnected to the transmission port 320 (S530). Then, if the number ofvacant entries in the output buffer 340 is smaller than a designatedexcessive congestion reference value, which is smaller than thereference value, the congestion determination section 410, whendetermining whether congestion has occurred, determines that excessivecongestion has occurred (S540). And in this case, since excessivecongestion has been detected (YES at S540), the transmission intervalcontrol section 430 calculates a transmission interval (e.g., several mseconds to several hundreds of m seconds) that is longer than whenexcessive congestion has not been detected (S550).

In the event that excessive congestion is not detected (NO at S540), thetransmission interval control section 430 employs the bandwidth tocalculate a transmission interval (e.g., several m seconds to severalhundreds of m seconds) that is longer than when congestion is notdetected and is shorter than when excessive congestion is detected(S560). Thereafter, the obtained transmission interval is designated, bythe transmission interval control section 430, for use by the controlsignal transmission section 440 (S570).

FIG. 7 is a graph showing a transmission interval that the controller370 changes in accordance with the number of vacant entries in theoutput buffer 340. In the example in FIG. 7, it is assumed that thebandwidth obtained by the bandwidth acquisition section 420 is aconstant, regardless of the number of vacant entries in the outputbuffer 340. On condition that the number of vacant entries in the outputbuffer 340 is equal to or greater than the reference value, to determinewhether congestion has occurred, the transmission interval controlsection 430 transmits a control signal at a predesignated transmissioninterval, regardless of the number of vacant entries in the outputbuffer 340.

On condition that the number of vacant entries in the output buffer 340is smaller than the reference value for the occurrence of congestion,the transmission interval control section 430 designates a transmissioninterval (e.g., several m seconds to several hundreds of m seconds) thatis longer than when congestion has occurred. Furthermore, thetransmission interval controls section 430 extends the transmissioninterval on condition that the number of entries in the output buffer340 is reduced, or shortens the transmission interval on condition thatthe number of entries is increased. Thus, on condition that the numberof vacant entries is smaller than the reference value, the transmissioninterval control section 430 designates a transmission interval that islonger than when excessive congestion is not detected. Preferably,compared with when excessive congestion is not detected, thetransmission interval control section 430 increases the incrementallength of the transmission interval relative to the reduction in thenumber of vacant entries in the output buffer 340.

As described above, the transmission interval control section 430designates a long transmission interval on condition that congestion isdetected, and changes the incremental length of the transmissioninterval in accordance with the level of congestion. As a result, thetransmission interval control section 430 can control the transmissioninterval, so that a relationship between the transmission interval andthe number of vacant entries at the output buffer 340 is always amonotone decreasing function represented by a concave curve. Therefore,even when congestion is detected, the transmission interval can beappropriately controlled, the deterioration attributable to congestioncan be prevented, and the congestion can easily be removed.

The present invention has been explained by employing the embodiment;however, the technical scope of the invention is not limited to thisembodiment. Further, it will be obvious to one having ordinary skill inthe art that the embodiment can be variously modified or altered.Therefore, embodiments that include such modifications or alterationsare also included within the technical scope of the present invention.

1. A communication relay apparatus having a plurality of communicationports, comprising: a communication relay section, including at least onereceiving side communication port which receives communication data froma transmission side apparatus and at least one transmitting sidecommunication port which transmits the communication data to a receivingside apparatus, said communication relay section transmitting thecommunication data received from the transmission side apparatus throughsaid receiving side communication port to a receiving side apparatusthrough said transmitting side communication port; a buffer beingconnected with said transmission side communication port for storing thecommunication data before the transmission to the receiving sideapparatus through said transmitting side communication port; a controlsignal transmitting section for transmitting, in response to atransmission of the communication data from said buffer, a controlsignal representing a status in which an entry of said buffer isavailable to the transmitting side apparatus at a predeterminedtransmission interval; a bandwidth information acquiring section foracquiring the bandwidth of the communication line connecting to saidtransmission side communication port; and a transmission intervalcontrol section for controlling said transmission interval in accordancewith the bandwidth acquired by said bandwidth information acquiringsection.
 2. A communication relay apparatus according to claim 1,further comprising: a plurality of said receiving side communicationports for which communication priorities are designated in advance,wherein said transmission interval control section permits said controlsignal transmitting section to transmit said control signal to areceiving side communication port having a higher communication priorityat a shorter transmission interval than a receiving side communicationport having a lower communication priority.
 3. A communication relayapparatus according to claim 1, wherein said transmission intervalcontrol section shortens said transmission interval on condition thatvacant entries included in said buffer are increased.
 4. A communicationrelay apparatus according to claim 3, wherein said transmission intervalcontrol section controls said transmission interval so that arelationship between said transmission interval and the number of vacantentries in said buffer is a monotone decreasing function shown as aconcave curve.
 5. A communication relay apparatus according to claim 1,further comprising: a congestion determination section for determining,on condition that the number of vacant entries in said buffer is smallerthan a reference value, that communication congestion has occurred atsaid communication relay apparatus, wherein, on condition thatcongestion has occurred, said transmission interval control sectiondesignates said transmission interval based on said bandwidth, or oncondition that congestion has not occurred, designates a predesignatedtransmission interval that is shorter than said transmission interval.6. A communication relay apparatus according to claim 5, wherein, incase the number of vacant entries in said buffer is smaller than saidreference value, said congestion determination unit determines, oncondition that the number of vacant entries is smaller than an excessivecongestion reference value that has been predesignated and is smallerthan said reference value, that excessive congestion has occurred; andwherein, on condition that excessive congestion has occurred, saidtransmission interval control section designates a longer transmissioninterval than a transmission interval designated in case excessivecongestion has not occurred.
 7. A communication relay apparatusaccording to claim 1, further comprising: a notification section fornotifying said transmission side apparatus of a predesignated smallervalue than a capacity of said buffer that indicates the maximum quantityof communication data that can be sequentially transferred to saidcommunication relay apparatus.
 8. A communication relay apparatusaccording to claim 1, wherein said communication relay section transmitscommunication data, which have been received through said receiving sideport from a specific apparatus used for a fiber channel, to anotherapparatus used for a fiber channel, across a communication line having asmaller communication bandwidth than a communication line extended tosaid specific apparatus; wherein said control signal transmittingsection transmits an R_RDY signal for a fiber channel as said controlsignal; wherein said bandwidth information acquiring section obtains abandwidth for said communication line extended to said another apparatusused for a fiber channel; and wherein said transmission interval controlsection employs said bandwidth to designate a transmission interval forsaid R_RDY signal.
 9. A communication relay apparatus according to claim1, wherein said transmission interval control section extends saidtransmission interval on condition that vacant entries included in saidbuffer are reduced.
 10. An information management system comprising: acommunication relay apparatus including a plurality of communicationports; and a storage device connected to communication ports of saidcommunication relay apparatus, so that communication data that aretransmitted to external apparatuses are relayed by said communicationrelay apparatus, wherein said communication relay apparatus includes acommunication relay section, including at least one receiving sidecommunication port which receives communication data from a storagedevice and at least one transmitting side communication port whichtransmits the communication data to a receiving side apparatus, a bufferbeing connected with said transmission side communication port forstoring the communication data before the transmission to the receivingside apparatus through said transmitting side communication port, acontrol signal transmitting section for transmitting, in response to atransmission of the communication data from said buffer, a controlsignal representing a status in which an entry of said buffer isavailable to the storage device at a predetermined transmissioninterval, a bandwidth information acquiring section for acquiring thebandwidth of the communication line connecting to said transmission sidecommunication port, and a transmission interval control section forcontrolling said transmission interval in accordance with the bandwidthacquired by said bandwidth information acquiring section; and whereinsaid storage device includes a recording section for recording data, anda transmission section for, in response to receipt of said controlsignal from said communication relay apparatus, transmitting to saidcommunication relay apparatus data stored in said recording section. 11.The information management system according to claim 10, wherein saidcommunication relay apparatus further comprises: a plurality of saidreceiving side communication ports for which communication prioritiesare designated in advance, wherein said transmission interval controlsection permits said control signal transmitting section to transmitsaid control signal to a receiving side communication port having ahigher communication priority at a shorter transmission interval than areceiving side communication port having a lower communication priority.12. The information management system according to claim 10, wherein insaid communication relay apparatus, said transmission interval controlsection shortens said transmission interval on condition that vacantentries included in said buffer are increased, or extends saidtransmission interval on condition that vacant entries included in saidbuffer are reduced.
 13. The information management system according toclaim 10, wherein in said communication relay apparatus, saidtransmission interval control section controls said transmissioninterval so that a relationship between said transmission interval andthe number of vacant entries in said buffer is a monotone decreasingfunction shown as a concave curve.
 14. The information management systemaccording to claim 10, wherein said communication relay apparatusfurther comprises: a congestion determination section for determining,on condition that the number of vacant entries in said buffer is smallerthan a reference value, that communication congestion has occurred atsaid communication relay apparatus, wherein, on condition thatcongestion has occurred, said transmission interval control sectiondesignates said transmission interval based on said bandwidth, or oncondition that congestion has not occurred, designates a predesignatedtransmission interval that is shorter than said transmission interval.15. The information management system according to claim 14, wherein, incase the number of vacant entries in said buffer is smaller than saidreference value, said congestion determination unit determines, oncondition that the number of vacant entries is smaller than an excessivecongestion reference value that has been predesignated and is smallerthan said reference value, that excessive congestion has occurred; andwherein, on condition that excessive congestion has occurred, saidtransmission interval control section designates a longer transmissioninterval than a transmission interval designated in case excessivecongestion has not occurred.
 16. The information management systemaccording to claim 10, wherein said communication relay apparatusfurther comprises: a notification section for notifying saidtransmission side apparatus of a predesignated smaller value than acapacity of said buffer that indicates the maximum quantity ofcommunication data that can be sequentially transferred to saidcommunication relay apparatus.
 17. The information management systemaccording to claim 10, wherein said communication relay sectiontransmits communication data, which have been received through saidreceiving side port from a specific apparatus used for a fiber channel,to another apparatus used for a fiber channel, across a communicationline having a smaller communication bandwidth than a communication lineextended to said specific apparatus; wherein said control signaltransmitting section transmits an R_RDY signal for a fiber channel assaid control signal; wherein said bandwidth information acquiringsection obtains a bandwidth for said communication line extended to saidanother apparatus used for a fiber channel; and wherein saidtransmission interval control section employs said bandwidth todesignate a transmission interval for said R_RDY signal.
 18. A controlmethod for a communication relay apparatus that includes a plurality ofcommunication ports, a communication relay section, including at leastone receiving side communication port which receives communication datafrom a transmission side apparatus and at least one transmitting sidecommunication port which transmits the communication data to a receivingside apparatus, and a buffer being connected with said transmission sidecommunication port for storing the communication data before thetransmission to the receiving side apparatus through said transmittingside communication port, comprising the steps of: transmitting, inresponse to a transmission of the communication data from said buffer, acontrol signal representing a status in which an entry of said buffer isavailable to the transmitting side apparatus at a predeterminedtransmission interval; acquiring the bandwidth of the communication lineconnected to said transmission side communication port; and setting saidtransmission interval in a transmission interval control section inaccordance with the bandwidth that is acquired.
 19. The control methodfor a communication relay apparatus according to claim 18, comprisingthe further step of: providing a plurality of said receiving sidecommunication ports for which communication priorities are designated inadvance, wherein said transmission interval control section permits saidcontrol signal transmitting section to transmit said control signal to areceiving side communication port having a higher communication priorityat a shorter transmission interval than a receiving side communicationport having a lower communication priority.
 20. The control method for acommunication relay apparatus according to claim 18, comprising thefurther step of shortening said transmission interval on condition thatvacant entries included in said buffer are increased.
 21. The controlmethod for a communication relay apparatus according to claim 20,comprising the further step controlling said transmission interval sothat a relationship between said transmission interval and the number ofvacant entries in said buffer is a monotone decreasing function shown asa concave curve.
 22. The control method for a communication relayapparatus according to claim 18, comprising the further steps of:providing a congestion determination section for determining, oncondition that the number of vacant entries in said buffer is smallerthan a reference value, that communication congestion has occurred atsaid communication relay apparatus, wherein, on condition thatcongestion has occurred, said transmission interval control sectiondesignates said transmission interval based on said bandwidth, or oncondition that congestion has not occurred, designates a predesignatedtransmission interval that is shorter than said transmission interval.23. A computer program stored on a computer readable medium that makesan information processing apparatus function as a communication relayapparatus having a plurality of communication ports, wherein thecommunication relay apparatus includes a plurality of communicationports, a communication relay section, including at least one receivingside communication port which receives communication data from atransmission side apparatus and at least one transmitting sidecommunication port which transmits the communication data to a receivingside apparatus, and a buffer being connected with said transmission sidecommunication port for storing the communication data before thetransmission to the receiving side apparatus through said transmittingside communication port, comprising: means for transmitting, in responseto a transmission of the communication data from said buffer, a controlsignal representing a status in which an entry of said buffer isavailable to the transmitting side apparatus at a predeterminedtransmission interval; means for acquiring the bandwidth of thecommunication line connected to said transmission side communicationport; and means for setting said transmission interval in a transmissioninterval control section in accordance with the bandwidth that isacquired.
 24. The computer program according to claim 23, furthercomprising: means for providing a plurality of said receiving sidecommunication ports for which communication priorities are designated inadvance, wherein said transmission interval control section permits saidcontrol signal transmitting section to transmit said control signal to areceiving side communication port having a higher communication priorityat a shorter transmission interval than a receiving side communicationport having a lower communication priority.
 25. The computer programaccording to claim 23, further comprising means for shortening saidtransmission interval on condition that vacant entries included in saidbuffer are increased.
 26. The computer program according to claim 25,further comprising means for controlling said transmission interval sothat a relationship between said transmission interval and the number ofvacant entries in said buffer is a monotone decreasing function shown asa concave curve.
 27. The computer program according to claim 23, furthercomprising: means for providing a congestion determination section fordetermining, on condition that the number of vacant entries in saidbuffer is smaller than a reference value, that communication congestionhas occurred at said communication relay apparatus, wherein, oncondition that congestion has occurred, said transmission intervalcontrol section designates said transmission interval based on saidbandwidth, or on condition that congestion has not occurred, designatesa predesignated transmission interval that is shorter than saidtransmission interval.